CS 268: End-Host Mobility and Ad-Hoc Routing Ion Stoica March 13, 2006

2 Overview  Wireless  End-host mobility

3 Wireless  Wireless connectivity proliferating -Satellite, line-of-sight microwave, line-of-sight laser, cellular data (CDMA, GPRS, 3G), wireless LAN (802.11a/b), Bluetooth -More cell phones than currently allocated IP addresses  Wireless  non-congestion related loss -Signal fading: distance, buildings, rain, lightning, microwave ovens, etc.  Non-congestion related loss  -Reduced efficiency for transport protocols that depend on loss as implicit congestion signal (e.g. TCP)

4 Problem Time (s) Sequence number (bytes) TCP Reno (280 Kbps) Best possible TCP with no errors (1.30 Mbps) 2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN (from Hari Balakrishnan)

5 Solutions  Modify transport layer  Modify link layer protocol  Hybrid

6 Modify Transport Protocol  Explicit Loss Signal -Distinguish non-congestion losses -Explicit Loss Notification (ELN) [BK98] -If packet lost due to interference, set header bit -Only needs to be deployed at wireless router -Need to modify end hosts -How to determine loss cause? -What if ELN gets lost?

7 Modify Link Layer  Advantages: -Limited changes: only link-layer affected -Preserve end-to-end (TCP) semantics  Three types of losses -Total packet loss -Partial packet loss -Packet corrupted by bit errors  Three methods to reduce packet loss -Packet retransmission -Forward error correction -Packet shrinking

8 Retransmission  Advantages: -Optimal overhead: only lost packets are retransmitted  Disadvantages: “nasty” interactions between TCP control loop and link-level retransmission -Both TCP and link-layer can retransmit same packets -Can introduce packet reordering -Can introduce highly variable delays

9 FEC  Forward Error Correction (FEC) codes -k data blocks, use code to generate n>k coded blocks -Can recover original k blocks from any k of the n blocks -n-k blocks of overhead -Trade bandwidth for loss -Can recover from loss in time independent of link RTT Useful for links that have long RTT (e.g. satellite) -Pay n-k overhead whether loss or not Need to adapt n, k depending on current channel conditions

10 FEC & Packet Shrinking  Advantages: -No changes at end hosts or base-stations above link- layer -Decrease packet loss -Do not introduce variability  Disadvantages: -Overhead can be quite high, e.g., packet segmentation/reassembly, encoding/decoding

11 Flex [Eckhardt &Steenkiste ’98]  Combine the three types of error control  seven policies (three fixed and four adaptive)  Most sophisticated : Flex -When two or more packets in a window of ten are truncated  reduces “safe” packet size by 15% -When three consecutive packets do not experience truncation  linearly increase packet size -When two or more packets in a window of ten cannot be decoded  decrease user data by 15% (more conservative coding) -When three consecutive packets can be decoded  increase user data linearly  Note: adaptation exhibits a linear-increase multiplicative-decrease behavior

12 Hybrid: Indirect-TCP [Bakre & Badrinath ’94]  Split TCP connection into 2 TCPs  Advantages -Optimize performance for wireless TCP -No changes to protocol for fixed hosts (transparent to fixed hosts)  Disadvantages -Violate end-to-end TCP semantics (why?) -High overhead, because dual stack at BS -Might introduce high delays because packet buffering Fixed Host (FH) Base Station (BS) Mobile Host (MH) wireless TCP Internet regular TCP

13 Hybrid: Snoop-TCP [Balakrishnan et al. ’95]  Insert a “snoop agent” between fixed host (FH) and mobile host (MH) -Monitor traffic, retransmit packets and discard acknowledgements  Notes: -Avoid violating end-to-end semantics -What about layering? Fixed Host (FH) Base Station (BS) Mobile Host (MH) Internet TCP packet retransmissions

14 Overview  Wireless  End-host mobility

15 Motivation and Problem  Network Layer mobility -Movement = IP address change  Problem: -Location I take my cell phone to London How do people reach me? -Migration I walk between base stations while talking on my cell phone I download or web surf while riding in car or public transit How to maintain flow?

16 Solutions  Mobile IP (v4 and v6)  TCP Migrate  Other solutions

17 Mobile IP  Use indirection to deal with location and migration  Point of indirection: Home Agent (HA) -Resides in Mobile Host’s (MH) home network -Uses MH’s home IP address -As MH moves, it sends its current IP address to HA  Correspondent Host (CH) contacts MH through HA  HA tunnels packets to MH using encapsulation  MH sends packets back to CH -Tunnels packets back to HA (bi-directional tunneling) -Sends directly to CH (triangle routing)

18 Mobile IP Properties  Advantages -Preserves location privacy -CH does not have to be modified  Disadvantages -Triangle routing and especially bidirectional tunneling increase latency and consume bandwidth -HA is single point of failure

19 Mobile IP Route Optimization  CH uses HA to contact MH initially  MH sends its location directly back to CH  CH and MH communicate directly  Lose location privacy  CH must be modified

20 TCP Migrate [SB00]  Location: uses dynamic DNS updates -When MH moves to new IP address, it updates its home DNS server with new hostname to IP address mapping  Migration: -When MH moves, it sends update to CH  Advantage -No new infrastructure -Incremental deployable -Efficient routing  Disadvantages -Only works for TCP -Both CH and MH need new TCP implementation -No location privacy

21 i3 Based Mobility (Z+03)  Receiver R maintains a trigger (id, R) in the i3 infrastructure; sender sends packets to id  Advantages -Support simultaneous mobility -Efficient routing: receiver can chose id to map on a close i3 server -Ensure privacy  Disadvantage -Require a new infrastructure Sender Receiver (R1) Receiver (R2) idR1 idR2

22 Other solutions  Network specific mobility schemes -Cellular phones, b -Cannot handle mobility across networks (e.g. move laptop from cell phone to b) or between same network type in different domains (e.g. laptop from Soda Hall b to campus b)  Other mobility models -Terminal/personal mobility: e.g., accessing through IMAP from different computers -Session mobility: e.g., talking on cell phone, transfer call in progress to office phone

23 Summary  Not that important today -Few portable, wireless IP telephony devices -Cell phones have their own network-specific mobility schemes -IP-based wireless networks are not ubiquitous enough to be seamless  Future -Cellular networks will become IP-based, need IP mobility scheme -PDA are becoming more powerful

24 Project Presentations  Eight minutes  Five slides: -1 st slide: Title -2 nd slide: motivations and problem formulation Why is the problem important? What is challenging/hard about your problem -3 rd slide: main idea of your solution -4 th slide: status -5 th slide: future plans and schedule

25 Presentation Schedule (Wed 3/15)  Kirsten Chevalier  Li Guan & Cindy Song  Halldor Isak Gylfason  Chris Baker, Daekyeong Moon & Jorge Ortiz  Arsalan Tavakoli  Zhangxi Tan, Wei Xu & Xiaofan Jiang  Young Yoo  Lilia Gutnik, Ted, Vijay

26 Presentation Schedule (Mon 3/20)  Peter Bodik  Andrey Ermolinksiy, Daniel Chen, and Hovig Bayandorian  Youwei Zhang and Libin Jiang  Hemang Patel and Emad Salman  Jeremy Rahe  Artur Rivilis  Mao Ye at al  Jay Taneja  Padmanabhan Vasu, Mark, Kye